A (Faster) Journey To The Center Of The Earth

The latest Total Recall film (the one without Schwarzenegger) features the Fall, a tunnel through the Earth’s core. It’s an unlikely SF idea, but one that regularly comes up in physics classes, where students calculate how long the trip would take if the only force were gravity. The standard answer, 42 minutes, has just been overturned.

The McGill University physicist (whose usual research area involves measuring how squishy DNA is because “The physics of squishy things is interesting”) managed to speed up the journey to (and from) the center of the Earth by factoring in the densities of the planet’s different layers.

The destruction of the Fall in 'Total Recall'

“This was a bit of a surprise to me,” Dr Klotz said. “The way the Earth is structured, the gravity increases slightly as you go deeper towards the dense core, to about 110 per cent of its surface value, before getting weaker as you move through the core, reaching zero at the centre.”

The idea of tunnelling from one side of the planet to the other, commonly called a gravity train, was first proposed by British physicist Robert Hooke in a letter to Sir Isaac Newton. It surfaced again in a proposal to the Paris Academy of Sciences in the 19th Century and was rediscovered in 1966 when Paul Cooper published “Through the Earth in Forty Minutes”, also in the American Journal of Physics.

The basic idea is that a capsule dropped into a shaft would accelerate due to gravity all the way to the center, and then decelerate due to gravity all the way to the final terminus. Not only would it get you to the other side of the planet faster than the International Space Station completes half an orbit, but such a trip would require no energy.

Well, except for the air conditioning.

And that would be a significant expense. The center of the Earth is an estimated 5,400C (9,750F).

Landmarks on the way to the Earth's Core (Credit: Wikipedia)

The tunnel construction would be even more of a problem. As well as the heat, you’d have to battle pressures three million times the atmosphere at sea level.

To make matters worse, you’d have to pass through the liquid outer core, where currents within the molten rock would be trying to bend your shaft off course.

And finally there’s the problem of friction, which would slow the trip so that extra energy would be required to complete the journey.

You could in theory evacuate the tunnel, and use magnetic guides to keep the train from hitting the walls, but both pumping out the air and running the magnets would in practice require energy.

Gravity trains could, however, be a workable technology on solid bodies with no atmosphere, such as the Moon, should there ever be enough people on opposite sides who want to travel back and to.